Stabilized generator



Feb. 16, 1954 Filed Feb. 13, 1948 STABILIZED GENERATOR l L. E. NORTON 3 Sheets-Sheet l refe We# PWER ZoweZZEfo :Snventor I ttorneg Feb. 16, 1954 E. NORTON 2,669,659

STABILIZED GENERATOR Filed Feb. 15, 1948 5 sheets-sheet 2 nventor ZowellENonZ'on 'llllll" IIIILI dawn- Gttorneg Inn Feb. 16, 1954 E NORTON 2,669,659

STABILIZED GENERATOR Filed Feb. 13, 1948 s sheets-sheet 5 Patented Feb. 176, 1954 STABILIZED GENERATOR Lowell E. Norton, Princeton Junction, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application February 13, 1948, Serial No. 8,246

'This invention relates to methods andy systems for utilizing selected absorption lines of the same gas or different gases to produce a low frequency standard of high precision unaffected by temperature, pressure and other conditions which impair the accuracy of previously known low frequency standardssuited, for example, for driving of electric clocks or similar indicators of time or pressure.

The microwave absorption spectra of certain gases, includingammcnia, carbonyl sulphide and methyl halides, comprise spectra lines of distinctive and different frequency distribution for the different gases. At very low pressures, these lines or absorption regions may break up into a plurality of sharply defined lines, each correspending with a precise microwave frequency which is not affected by temperature, pressure, time, and other familiar variables.

Generally in accordance with Vthe present invention, the frequencies of two microwave oscillators are'stabilized with respect to a selected pair of aforesaid molecular resonance lines, and the outputs of these oscillators are mixed to produce a difference frequency which lis precisely determined by the selected gas-absorption lines, and which is low enough in the frequency spectrum to'be divided, if desired, in accordance with known techniques to a still lower frequency suited for operation of a clock or similar indicator designed to operate at low or power line `frequencies.

Preferably, `and `more particularly, each of the stabilized microwave oscillators is regulated by the error-voltage output of a phase comparator which detects any shift in the phase relation between two waves, one of which is produced by impressing upon a gas cell the output of a search oscillator periodically varying over a range including the frequency of the selected line of a molecular resonant gas, and the other of which is produced by the beat frequency between the search oscillator and the stabilized oscillator.

More specifically, the aforesaid waves are differentiated to produce two trains of pulses: the

pulses of onetrain coincide with occurrence of the maxima of the output wave of a frequency selector .upon which the aforesaid beat frequency is impressed, and the pulses of the other train are those resulting` from differentiation of the output wave of the gas cell. In consequence, the errorvol'tage produced by the phase-comparator always changes in one sense when the beat frequency maxima advance in phase and always changes in reverse sense 'when the beat frequency maxima lag in phase.v

6 Claims. (Cl. Z50-36) In one form of the invention, the selector used for discriminating in favor of a predetermined beat frequency, in the entire frequency interval of beat frequencies between the search and stabilized oscillators is sharply tuned, and the one and the same sharply tuned selector is alternately switched into the respective control systems of the two stabilized oscillators thus to eliminate from their aforesaid difference-frequency any remnant error otherwise probable because of the dissimilarity` in the eiiects of temperature and other variables upon the response characteristics of sharply tunedselectors individual to the two control systems.

In another form of the invention, high precision of the different frequency of the oscillators is obtained without the circuit complications necessarily involved in the aforesaid switching of a sharply tuned selector. In this modification, the frequency selectors are low-pass filters which need be only approximately similar because, regardless of the effects of temperature and the like, their outputs are each always symmetrical about the zero beat frequency between the corresponding search and stabilized oscillators.

The invention further resides in methods andl systems having features hereinafter describedl and claimed.

For a more detailed understanding the invention, and for illustration ofvarious systems embodying and utilizing it, reference ismade to the accompanying drawings in which:

Figure 1 is a block diagram of a system utilizing stabilized microwave oscillators to produce a low standard-frequency;

.Figures 2 to '7 inclusive are schematic diagrams of circuit components of blocks of Figurefl; and

Figure 8 is a block diagram of a modification of the system of Figure 1.

It i's known there are a number of gases including NH3, COS, Cl-laOH, CHaNl-Iz and SO2 which exhibit-selective absorption of microwave energy. From measurements of the molecular resonance characteristics of such a gas, it is known that the magnitude of the absorption coefficient may loe-quite independent of the gas pressure, but that the width of the absorption region decreases substantially linearly with decrease of pressure. Specifically, at a wavelength of 1.25 centimeters (24.0 kilomegacycles), the Q of the 3, 3 line of ammonia is approximately 10 at a gas pressure'of of an atmosphere; is 100 at l/o@ of an atmosphere, etc. However, as the pressure is 'further and further reduced, for example.' tothe order of tenths of a millimeter of mercury,

or substantially less, the absorption region breaks up into a plurality of sharply defined lines, each precisely corresponding with a particular microwave frequency unaffected by usually encountered ambient conditions, such as temperature, pressure and the like and having a Q of the order of 50,000 and higher.

Generally in accordance with the methods used in the practice of the present invention, a selected pair of these sharply defined lines, which may be of the same or of different gases, are utilized in stabilization of two microwave oscillators Whose output frequencies are mixed to produce a difference-frequency which may be of the order of tens of megacycles and is precisely related to the difference between two microwave frequencies corresponding with vthe selected gas absorption lines.

Csi

The oscillator difference-frequency so produced may be impressed upon a conventinal form of frequency divider, such as a multivibrator, suited for operation at frequencies of this lower order.

to produce an output frequency which may be of the order of cycles per second and so suited for energiza'tion of an electric clock or other indicator calibrated in terms of time or frequency. `Each of the aforesaid microwave oscillators may be stabilized at a selected molecular resonant frequency of a gas by recourse to any of the methods or systems disclosed in copending applications, Serial No. 786,736, filed November 18, 1947; Serial No. 1,240, f'lled January 8, 1948; Serial No. 4,497, led January 27, 1948; Serial No. 5,563, led January 31, 1948; and Serial No. 5,603, filed January 31, 1948, but for completeness and clarity in the two preferred systems herein specifically shown and described, there are schematically shown preferred stabilizing arrangements which afford a wider range of control without possibility of ambiguous interpretation of the sense of deviation of the stabilized oscillator frequency.

Now referring to Figure l, the microwave oscillator IIIA is stabilized at a frequency a/27r which is equal to a selected molecular resonance frequency of gas within cell I IC plus (or minus) the intermediate frequency F to which an amplifier I2 is sharply tuned. A second microwave oscillator IOB is stabilized at a frequency /21r which is equal to the molecular resonance of gas within cell IID plus (or minus) the same intermediate frequency (F). The outputs of the two stabilized microwave oscillators I A, I 0B are impressed upon the mixer I3, which may be and preferably is a crystal rectifier.

The two microwave frequencies a and may be and preferably are so chosen that their difference frequency appearing in the output of mixer I3, is low enough to be divided by a conventional frequencydivider circuit, generically represented by block I4 to produce an output frequency of the order of cycles per second. By way of example, assuming the gas used in the cells IIC and IID is ammonia, the 8, 5 line and the 9, 'I line correspending respectively with 20719.19 megacycles and 20735.47 megacycles may be selected to afford a difference frequency of 16.28 megacycles; the 5, 4 line (22,653 megacycles) and the 4, 3 line (22,683.73 megacycles) may be selected to afford a difference frequency of 30.73 megacycles; or the 2, 2 line (23,722.59 megacycles) and the E, I iine (23,694.48 megacycles) may be selected to afford a difference frequency of 24.11 megacycles,

4 and so on for other selected pairs of lines. By using different gases in the two cells, the number of possible pairs for selection is still further increased with corresponding increase in the precise frequency-differences which may be availed of in practice of the invention.

Each of the gas cells IIC, IID may be a section of waveguide having at each of its opposite ends a window of mica or other suitable dielectric permitting passage of microwave energy and forming a gas-tight enclosure suitable for containing gas at pressures of 0.02 millimeter of mercury or less.

The input or output of frequency divider I4 mayitself be used as a precision sub-microwave frequency standard or, when it is desired to operate a precision clock or indicator I5, the output of divider I4 may be amplified by a conventional low-frequency power amplifier I6 to drive the motor of clock i5 which by choice of the gas lines, of the dividing factor n and of the gearingl in the indicator may have, for example, a one revolution per second rotation. Preferably, the

'division ratio n is very large so that the output of the divider I4 is very close to one cycle per second andl any further ratio change is effected by selection of the gearing. It is thus insured that any remnant control error in the instantaneous difference between the frequencies a and of the stabilized oscillators IUA and IDB is very small and so affords a time indication of high precision unattainable by any previously known means.

Preferably the method of stabilizing each of i lator IIIA, suitably reduced if desired by attenuator I'IA, is impressed through the directional coupler ISA upon the mixer I3AC, preferably a crystal rectifier. (This and other directional couplers of systems herein disclosed may be of the type more fully described in copending application Serial No. 786,736.) The output of a frequency-modulated search oscillator IGC is also impressed upon mixer I 3AC through a directional coupler I8C preceded if desirable by an attenuator I'IC. The output of the-,search oscillator IIlC is periodically swept over a band of frequencies which may, for example, be 5 or 10 megacycles wide and which includes both the frequency a/21r, the molecular resonance frequency of gas in cell I IC, and the frequency a/21r plus (or minus, but not both) the intermediate frequency \///21r- The sweep frequency or repetition rate R of the search oscillator IIlC is low compared to the carrier frequencies of the oscillators IOA and IOB and is substantially different from the intermediate frequency ,b/21r: the search frequency may, for example, be 60 cycles per second as produced by any suitable electronic or mechanical modulator and the modulating waveform is preferably sawtoothed. For clarity of explanation, it will beassumed that the carrier frequency of oscillator IDC is periodicallyv 5 raised fromfan initial frequency below the. molecular resonant frequency of the gas in cell HC .to a frequency higher than frequency aww-PM2# and Vthen abruptly returned to the vinitial Afrequency which is higher l.than lfrequency The output of mixer I SAC rinciudes, :as :one vof its components, the frequency w/21r which is selected by amplifier I'ZRand:impresseduponrectier ISR to provide a series'of control pulsesfor a'switching potential generator'I 9 which controls the pairs of ampli'ers MA, ZI'Aand 29B, 2Il3=so that the .pairs are alternately operative. .The rst pair of amplifiers 2SA, `ZIA is connected in the I. F. channel ofthe microwave 'oscillator IDA stabilized at frequencya/Bw and the second "pair of ampliers 20B, ZI-B is connected in the same LF. channel (and used alternately) ofthe microwave oscillator IBB 'stabilized at'frequency /21r.

A suitable form of switching'potentialgenera-v tor I9, schematicallyfshown in Figure 2, ismore fully described in my U. S. Patent.2,422,`l22. 'In brief, the sweep-frequency impulses selected :by amplifier IZR, Figure 1, which occur in timebeforethe sweep frequency envelopes Afrom ampliiier I2, by suitable selection-ofrelativevalues of frequencies rtv/21| ando/2r, are applied 4tothe input circuit of an amplier tube 22 whose output signal is impressed on the grid circuit of the inverter and vpush-pull output tube 23 to produce two series of positive, substantially rectangular pulse signals a and b of complementary length. Thepulses a are applied through condenser 25A, Figures land 2,to ampliiers.`2IlA. 2IA eifectively to unblock them Vor turn kthem on for intervals of duration t1 during which the ampliiiers 25B and 2l Bare off': the pulses b are similarly appliedthrough condenserZ'AiB, Figures 1 and.2,to:ampliers. 2IlBandi2 IB 'eifectively to turn them onfor intervals of duration t2 during which the (amplifiers 2BA and rZIAare No",

During each time interval :t1 zfor which it is 011, the amplifier ZIA transmits from rectier ISF to an amplier-diierentiatorrepresented by block 25A in Figure 1,'a wave or pulse having maximum amplitude at the instant the dilference frequency between oscillator 1IIlA and the FM or search Aoscillator .H3C corresponds with frequency War to which amplier I2 is sharply l tuned. output of amplifier 21Ict'is apotential proportions-itc the envelope ofamplifler |=2 atrepetition ferquencyR: otherwise expressed the output of the amplifier I2 during intervals-t1 in cell IIC. More specifically, output energy from oscillator lliCis transmitted as oya'wave guide to gas cell IIC'through a path which may include an attenuator -CI-'I, and a matching transformer 26A, both of types suitable `forfopeiwation at microwave frequencies. 'Thernicrwave energy'selectively passedby the gas celliisrectie ed by vcrystal ISC producing pulses or waves having the same'repetition rate as the modulation-frequency R of oscillator ISC. For cancellation of amplitude-modulation yeffects andof other variables, output energy'from oscillator IUC is transmitted` by directional coupler `CI8 to a second crystal rectier yClt, or equivalent, -re` versely poled with respect to'rectier ISC. "The differential-output of the rectiiiers `I3C and CI'3 is impressed upon the diiferentiator-alnpliiler 25C.

A suitable typeof diiferentiation-ainplifiery cir-- cuit *25 ffcr 'each of the differentiato'reamplifiers 25A and 25C (as well as for differentiator-amplitiers 25B,`25D later discussed) `isshown in Figure 3. *The potentials proportional to the resonance curvesor `characteristics are impressed'upon the input circuit of tube 2l arevamplied, reversed. in polarity 'and ilnpressed'upon the Adifferentiator network comprising condenser 28an`d resistor 2B to apply to the'grid of tube Sil a doublefpulse dforeach'input pulse. Each pulse d has a'negai tive and a positive peak and lis very Ysteep between these peaks. Theresulting output of tube 3Q is a'series of sharp pulses c having a steep rise at the ycenter of the corresponding :original inputpulse to-tub-e 2'I.

"The time constant o'f lthe resistor-capacitor combination '23, 29 isv smallcompared to thercpe-` tition `period'i so that the input pulse totube 30 is diiferentiated. The-grid-'cathode'bias of tube 3u `is such that with'an anode resistance ofhigh value the anode is only slightly'above-'cathode potential. Consequently, the firstpositive'swing of the grid-cathodepotential of tube lllprodu'ce'sv little change in'anode `potential whereas the one immediately following 'a 'steep 'negative A`swing causes a 'steeppositive swing in anode potential. Thissteep positive rise may be used to trigger a subsequent'gas ltriode, 'later hereinmentioned, 'so that 'the output'pulses of'the trode Voccur'atthe center or peak of 'the intermediate frequencyenvel-ope from beat-frequency selector AI2 or of `the absorption envelope of cell VI IC depending upon the control channel'under consideration.

The output pulses eA, Figure '1, of amplifiercliierentiator 25A are applied to vva pushepullimpulse generator BIA to produce two trains `o'i' pulses Aof `the same repetition frequency `as .pulses ci`- so that for each 'inputpulse eA,'there'is pro` duceda pairoi sharp output pulses, fN, fr., concurrent in time and of opposite polarity. Two suitable ktypes of push-pull-iinpulse 'generator are shown in`Figures 4 and-"5. 'In the type shown inFl'gure 4, using a gas tiiotle 32 to whose grid the input `pulses ,are applied, the positive output pulses -fp Vappear across the cathode resistor "33 and the negative output pulsesffN appearacross resistor .34 .connected to the anode of the ltube bycondenser 35 and to the cathode of the tube by .resistor '33. Alternatively, vthe impulse-,generator may be of the two-tube type shown in Figure 5 in which .thetotaloutput voltage of tube 32 appears across the cathode resistor33 to alford positive output pulses f1, which areftwice .the amplitude obtainable with the circuit of Figure e, other conditions being the saine. -'lhecathode resistor 33 of Figure 5 vis tapped and `'part ofthe pulse output voltage applied to the Agrid 'of 1a second tube '351:0 produce thenegative pulseffN across resistor 34 in Aseries with condenser 35 in theoutput circuit thereof. .The ,circuitzconstants areso 'chosenthat these pulses .are .equalin amplitude to the negative pulses fN- and occur concurrently therewith so that as in Figure 4, each input pulse produces a pair of sharp output pulses of opposite polarity and coincident in time.

The output pulses ec, Figure 1, of amplifierdifrerentiator 25C are applied to a sawtooth wave generator 37A so that for each input pulse ec thereof there is a sawtooth output pulse sA having an abrupt rise and a linear decay. A suitable type of sawtooth generator is shown in Figure 6. The sharp input pulses eC are applied to the grid of tube 38 to produce across capacitor 39, which is in series with capacitor 4B between the anode and grid of the tube, a series of sawtooth pulse si. of the repetition rate R.

Reverting to Figure 1, the sharp pulses fN and f1, from the double impulse generator 3IA and the sawtooth pulses s A from the sawtooth generator 31A are applied to a phase-detector or phase-comparator 4IA for producing as later herein more fully described, changes in a D. C. error voltage whose sense and magnitude depend upon the sense and extent of deviation of the frequency A of oscillator IBA from its desired value. From the foregoing, it shall be understood that the time of initiation of each sawtooth pulse sA is rigidly related to the instant that the carrier frequency of FM oscillator IBC in each cycle of its modulating frequency R passes through the frequency of molecular resonance of gas cell HC. It shall further be understood that time relation between initiation of each pulse sA and the corresponding pair of pulses IN, f1, depends upon the instantaneous frequency A of stabilized oscillator IBA because if its frequency 1t/2W is above normal the difference frequency tlf/2", to which amplifier I2 is sharply selective, is attained earlier in the cycle of the modulating frequency R whereas if the frequency r exemplary of each of stabilized oscillators IBAV and IBB of Figure 1, by regulation of the D. C. voltage of the refiector anode 44. `A fixed D. C. voltage difference between the cathode 45 and cavity 46 of the klystron is maintained by a stable or stabilized supply generically represented by battery 4l. The voltage-difference between the refiector 44 and cathode 45 of the klystron depends uponl the I. R. drop across resistor 48 connected between the anode 49 of the modulator tube 5B and the positive terminal of a source of stabilized D. C. voltage generically represented by battery 5I; the gas discharge tube 52 and current-limiting resistor 53 provide additional constancy of the cathode-screen potential of tube 5B. The magnitude of the current through resistor 48 and therefore the reflector voltage depends upon the biasing voltage applied to the grid of modulator tube 5B: one component of this biasing voltage is of fixed value selected, for example, by adjustment of the potentiometer 54 supplied from battery 55 or other stable source. The other component of this biasing voltage is the error output voltage of the phase-detector 4I as appearing between terminals 56, 51 thereof.

In thel particular form of phase-comparator 8 4I shown in Figure 7, the positive and negative pulses f1, and fN from the preceding associated diferentiator-amplier are respectively applied to the anode of diode 58, or equivalent, and the cathode of a similar rectifier 59. The electrodes of the rectifiers are connected by resistors 5I, 6I whose common terminal 5l is one of the output terminals of the comparator; the other electrodes of these rectifiers are connected to the other output terminal 55 of the comparator. The pulses s from the preceding associated sawtooth generator are applied vto the anode of rectifier 58 through resistor 62 and condenser 63 and to the cathode of rectier 59 through resistor 64 and condenser 65. Thus, the average difference value of the current of sawtooth wave form which traverse the resistors 6I, 62 in each cycle of the repetition frequency R depends upon the phase relation of the sharp pulses fp, fN to the sawtooth pulse s and thus the grid-voltage of regulator tube 50 is corrected, if necessary, in each cycle of the modulating frequency R of the search oscillator to adjust the controlled or stabilized oscillator I B in sense to return its frequency to the desired value.

-By way of example, the following circuit constants are given as suitable for the phase-discriminator circuit 4I Capacitors:

63, 65 mfd-- .05 66, 61 mfd-- .01 68 mfd .0L-.25 69 v mfd 10 Resistors:

6 I megohms-- 1.8 62, 64 ohms-- 1,000 'IB ohms max-- 10,000

As thus far specifically described, the system provides for rigid control of the frequency of oscillator IBA by an error" voltage derived by comparison of the phase relation of pulses produced during the time intervals t1 for which the amplifiers 2BA and 2IA are on. During they intervening time intervals tz for which the amplifiers 2 IA and 2 IB are on, a generally similar arrangement, including the FM search oscillator IBD, gas cell IID, intermediate frequency amplifier I2, rectifier I3F, differentiator amplifiers 25B, 25D, impulse generator 3IB, sawtooth generator 31D, phase comparator 4IB, regulator 42B and variable power supply 43B is effective to stabilize the oscillator IBB for operation at a frequency 18/21t differing from the frequency of molecular resonance in gas cell I ID by the intermediate frequency F.

It is to be noted -that the same intermediate frequency amplifier channel I2, I3F is alternately used in control of oscillator IBA during intervals t1, and in control of oscillator IBB during intervals t2. Consequently, any effects of temperature or supply voltage, for example, upon the response characteristics of the intermediate frequency system cancel out so that in effect the difference-frequency of the stabilized oscillators is precisely related to the selected sharp molecular resonance frequencies, which are not affected by temperature, of the bodies of gas in cells IIC and I ID. The exact frequency of ,the input to amplifier i2 at which switching occurs is not critical so long as the switching pulse at crystal I3R occurs in the cycle of modulating frequency R before the crystal I3F receives a pulse either from amplifier 2BA or from amplifier 20B.

The modification shown in Figure 8 is in many respects similar to that of Figure 1 and consequently those elements of the two arrangements having corresponding functions are identified by the same reference characters. For brevity, only those elements or features differing from Figure 1 are specifically discussed.

With the simpler arrangement shown in Figure 8, there is avoided the circuit complications involved in switching of the sharply tuned I. F. amplifier I2, Figure 1, from one control channel to the other'. Instead of a sharply-tuned amplier i2 switched from one control channel to the other, there are used, in the system of Figure 8, two low pass filters IIZA, H2B individual to the two control channels. These two filters need be only approximately similar and may, for example, be suited to pass a band of frequencies from 0 to 100 kilocycles.

The reasons it is not necessary in such system to use a common, switched filter in order to obtain precision will appear from the following discussion. As the intermediate frequency output of rectifier ISAC (or ISBD) approaches and passes through zero-beat, the amplitude response of l the associated low-pass filter II2A (or II2B) Varies symmetrically about the carrier frequency of FM oscillator IEC which corresponds with zerobeat. The associated amplier-differentiator 25A (or 25B) therefore produces an output pulse which occurs at the center or zero-beat point of its input pulse regardless of any effect of temperature, for example, upon the band-width of the filter. In other respects, the operation of the systems of Figure 1 is similar to that of Figure 8 and it therefore appears unnecessary to repeat the description thereof.

It shall be understood the invention is not limited to the specific methods and systems illustrated and described and that changes and modifications may be made within the scope of the appended claims.

What is claimed is:

1. A system for producing a sub-microwave frequency of high precision which comprises two microwave oscillators, separate gas cells for stabilizing to substantially the same degree the fre- CII quency of each of said oscillators, said gas cells each exhibiting molecular resonance at respectively different frequencies whose difference corresponds with said sub-microwave frequency, and means for mixing the outputs of said stabilized oscillators to produce oscillations of said sub-microwave frequency.

2. A system in accordance with claim l in which the stabilizing means for each oscillator includes a search oscillator whose frequency is Varied over a range including the frequency of the stabilized oscillator and the frequency of one of said gas cells.

3. A system according to claim 2 including means for deriving beat frequency signals from said Search oscillator and one of said stabilized oscillators, a frequency selector providing an output which varies in amplitude with said beat frequency, and a phase comparator for detecting shifts in the phase relation between output signals from said frequency selector and one of said gas cells.

4. A system in accordance with claim l in which the mixer output is in turn impressed on a frequency-divider for production of a frequency of the order of cycles per second.

5. A system in accordance with claim 3 in which is` included a sharply-tuned beat-frequency filter and an electronic switch for alternately including said lter in rst one and then the other of said stabilizing means.

6. A system in accordance with claim 2 in which the stabilizing means includes a low-pass lter producing an output symmetrical about zero-beat of the search and stabilized oscillator frequencies.

LOWELL E. NORTON.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,398,694 Case Apr. 16, 1946 2,457,673 Hershberger Dec. 28, 1948 2,609,654 Hershberger Sept. 9, 1952 

